Pressure sensors for measuring pressure changes are generally known. Currently used micromechanical pressure sensors operate, for example, according to the piezoresistive method. Thus micromechanical sensors exist particularly as silicon pressure sensors, which are essentially made up of a diaphragm clamped on all sides, which arches in the event of a pressure difference between the two diaphragm surfaces. The signal conversion is performed, for example, using integrated monocrystalline or dielectrically insulated polycrystalline piezoresistors or by capacitance measurements with respect to a fixed counter-electrode (piezoresistive or capacitive signal conversion). Disadvantageous in these sensors is the fact that a high current is required for measuring the resistance, which results in an unnecessarily high power consumption, and that the piezoresistive effect is highly temperature-dependent, which causes problems especially in high-temperature applications.
Capacitive pressure sensors are therefore developed, in which pressure changes are detected via a capacitive evaluation of a diaphragm deflection. Thus, for example, C. Y. Lee et al., “Quartz Capsule Pressure Transducer for the Automotive Industry”, Society of Automotive Engineers, Inc. 1980, discloses a pressure sensor of this kind, in which a first quartz glass or sapphire substrate having a stationary electrode and a second quartz glass or sapphire substrate having a movable electrode are situated in such a way that the surfaces of the electrodes lie across from each other and that peripheral sections of the substrates are joined or bonded with each other by a glass having a low melting point such that a predetermined gap results in between.
German Patent No. DE 42 44 450 describes a method for manufacturing a capacitive pressure sensor that starts out from two substrates from the same electrically insulated material having surface sections that are situated in such a way that they lie across from each other and that are subjected to a joining process. In one of the substrates a recess is formed which is fitted with an electrode made of a conductive thin-layer. A second electrode, likewise made of a conductive thin-layer, is formed on a section of that surface of the second substrate that is to be joined with the first substrate. The two substrates are subsequently directly joined in such a way that the surfaces configured with the electrodes are laid opposite each other and are permanently joined to each other through the application of a heat treatment and that subsequently at least one of the substrates is reduced to an appropriate thickness. German Patent No. DE 35 20 064 describes a method for manufacturing a capacitive pressure sensor based on the principle that a layer made of insulating material, e.g. glass, is fused or cast onto a substrate made of a machinable and conductive material, e.g. silicon. Subsequently, a recess is produced in an electrically conductive component and the two components are hermetically joined in such a way that a chamber is produced between the two.
A silicon/glass/silicon pressure sensor construction, which uses capacitive changes to measure pressure changes, is also described in German Patent No. DE 689 13 177.
The above-mentioned capacitive pressure sensors, however, have the disadvantage that for contacting the electrodes configured as diaphragms processes are required which penetrate very deeply into the material of the substrate. This does not allow for simple implantations. In addition, wires near the surface leading to the evaluation electronics are required, which, however, are susceptible to parasitic capacitances, especially if the medium to be measured comes into contact with the side on which the lead wires are located. Since with the miniaturization of a capacitive sensor the capacitances to be measured become very small, much development expense must be spent on electromagnetic compatibility (EMC).
Moreover, conventional sensors and circuits made of silicon only reach maximum operating temperatures of below approx. 150° C. This is due to the band gap of silicon of 1.1 eV. At higher temperatures, the material increasingly loses its semiconductive properties. With the disappearance of the p-n junctions, diodes and transistors lose their function and the insulation of implanted resistors with respect to the substrate is suspended. Hence, pressure sensors have already been developed, which allow for operating temperatures of up to 200° C. through the use of the SOI (silicon on insulator) material system containing the layer sequence of a thin silicon surface layer and a thin electrically insulating silicon oxide on a thick silicon substrate. Such a sensor on the basis of the 3C—SiC—SOI material system is made up of an evacuated cavity which is closed on the upper side by a thin diaphragm. To this end, first a ring diaphragm is patterned in a plasma process from the lower side of the SOI wafer. The patterned SOI substrate is joined in a hermetically sealed manner to a second wafer using fusion bonding in vacuum such that the cavity has a remaining pressure of approx. 50 mbar. Subsequently, the SIC layer is grown on the upper side of the wafer bond, the piezoresistors are patterned and interconnected via TiWN/Au circuit traces (cf. E. Thielicke and S. Zappe; Technische Universität [Technical University] Berlin, Fachbereich [Department] 12—Elektrotechnik [Electrical Engineering], Institut für Mess—und Automatisierungstechnik [Institute for Measurement and Automation Engineering], Jahresbericht [Annual Report] 2000, Bericht [Report] 193, p. 44 ff., December 2000).